This application is the US national phase of International Application No. PCT/GB99/04407, filed Dec. 23, 1999, the entire content of which is hereby incorporated by reference.
Projection displays, eg for lecture theatres, often use electrically addressed Spatial Light Modulators (EASLM). A Spatial Light Modulator is a device that spatially rather than temporally modulates one or more components of an otherwise uniform beam of light; an EASLM can be defined as a spatial light modulator whose elements are connected to and addressed by an electrical circuit.
EASLM technology for projection displays is maturing quickly and there are various products on the market already; these are currently based on two different technologies:
Transparent TFT SLMs;
Digital Micromirror Device (DMD). See for example J M Younse, Projection Display Systems based on the Digital-Micromirror Device, Proceedings of the SPIE, Vol 2641, 1995.
Projection displays that use relatively large TFT-type EASLMs have been available for some time. However, they have all the disadvantages of this type of technology (expense, speed, fill factor etc). These disadvantages can be overcome by the use of VLSI silicon backplane based EASLMs which offer major improvements in the quality of the integrated circuitry and costs of manufacture. The DMD-based projection display is an example of such a system; however, this technology, although available, is still new and untried and there are doubts regarding its true costs.
A third type of display is possible using ferro-electric liquid crystals (FELC) over VLSI silicon backplanes; apart from cost and the type of modulation, the FELC technology is very similar to the DMD technology. However, there are also problems with this approach, some shared with the DMD technology-principally that very large image sizes and resolutions are difficult and expensive to achieve. For a description of this technology see, for example, N Collings et al; xe2x80x9cEvolutionary Developments of Advanced Liquid Crystal Spatial Light Modulatorsxe2x80x9d (Applied Optics Vol 28, No 22, Nov. 15, 1989) and references therein.
According to the present invention there is provided an image generating device comprising an electrically addressable spatial light modulator means capable of forming an image and an optically addressable spatial slight modulator means for producing an image and image transfer means for relaying the image from the electrically addressable spatial light modulator means to the optically addressable spatial light modulator means wherein the image produced by the optically addressable spatial light modulator means is formed from a plurality of successive images formed by the electrically addressable spatial light modulator means.
An Optically Addressable Spatial Light Modulator (OASLM) is a spatial light modulator each of whose elements is optically non-linear in such a way that illumination by one ray of light will influence the action of the element on a second ray of light. Because it is addressed optically it can include a highly planar reflective surface for projection applications. The EASLM will generally be pixellated, for instance an array of liquid crystal elements, addressed by suitable, eg orthogonal, electrodes or circuitry. Using the EASLM merely as a pixel generator and producing the final image from an OASLM, which forms a high quality optical front end, has various advantages:
Comparatively simple and cheap techniques can be used to make OASLMs;
The size and optical quality of an OASLM is greater than that possible for all but prohibitively expensive EASLMs (an OASLM need have no topography underlying the pixels and can use very high quality cold-deposited mirrors); moreover protection displays require an image of larger area than is economically feasible with active-backplane EASLMs;
Additional processing of the image can be done by the transfer optics (see below); and
High intensity illumination of the EASLM is avoided.
The EASLM device also need only operate with monochromatic light, even for a frame sequential colour display system. The performance of the EASLM and the optical transfer means can therefore be optimised for this wavelength of operation, permitting the exploitation of low cost diffractive and holographic optical components.
For a description of OASLMs see below and also D Williams et al, xe2x80x9cAn Amorphous Silicon/Chiral Smectic Spatial Light Modulatorxe2x80x9d, Joint of Physics D, Vol 21, 1988.
As an EASLM can be operated at addressing rates in excess of what would be required for image display the EASLM can be used at a high addressing rate to generate subsequent parts of final image to be produced on the OASLM. Each successive image produced by the EASLM may be written to a separate part of the OASLM which is driven at a suitable addressing rate for display. This allows an increase in the resolution of the display system without needing an increase in the size of the EASLM.
The image from the EASLM may be imaged sequentially on different parts of the OASLM in order to build up the final image. The image transfer means may supply an image from the EASLM to one or more parts of the OASLM at any one time. The image transfer means could comprise beam steering apparatus to steer the image to the desired part of the OASLM. The beam steering apparatus could be any suitable apparatus for moving the image to the desired part of the OASLM as would be readily appreciated by one skilled in the art. Alternatively the EASLM and image transfer means could comprise an array of transfer means for relaying the image to different parts of the OASLM, each transfer means in the array having a means for preventing the image being transferred to that part of the OASLM when not required.
In a preferred embodiment the image from the EASLM is simultaneously imaged onto a plurality of different parts of the OASLM and only part of the OASLM is addressed to receive an image. This has the advantage of allowing simple and fixed image transfer means, such as an array of lenses, to relay the image from the EASLM to the OASLM. As only part of the OASLM is addressed however only that part of the OASLM will receive that image. When the next, image is displayed on the EASLM this new image will again be imaged a plurality of times on the OASLM. A different part of the OASLM will be addressed however and thus this part receives the image from the EASLM.
In a further preferred embodiment the OASLM will have an ability to store or retain the image segments across the device until the complete image has been written. This may be achieved using charge trapping in the photosensor, display modes with intrinsic memory or transient response times within the OASLM such as will be readily understood by one skilled in the art. The higher complexity image can then be read out with without image decay problems. The data held on the OASLM may then be erased and new data written or the old data may be repeatedly readout.
Preferably the EASLM incorporates an active backplane and thus includes an array of crystalline silicon transistors addressing a liquid crystal layer so as to produce the EASLM image, and the OASLM is a layer of liquid crystal and a layer of photo-sensor sandwiched between a pair of transparent conductors. In order to decouple the modulating light incident on the OASLM from the output light a mirror layer can be included between the liquid crystal and photosensor layers. This may formed by a pixelated metal layer or a multi-layer Bragg reflector type mirror. The inclusion of light blocking layers within the OASLM device may also be advantageous to the system performance.
Advantageously the image transfer means may be adapted to process the image from the EASLM. The transfer means may thus include between the EASLM and OASLM an optical information processor; this processor may be for example a hologram, such as an etched glass plate, as is known. The use of a hologram allows a multiplication of the size and of the resolution of the image. The circuitry that is present in practice to co-ordinate the EASLM and the OASLM so that the image projected onto the OASLM is spatially (de-)multiplexed may then also control the optical information processor.
In the rather simpler case where incoherent light is used the image transfer means can be simply a lens system transferring the EASLM image, preferably with magnification, to the OASLM. Further extension in resolution enhancement can also be achieved by using multiple EASLMs.
Preferably the image display system is a colour display system wherein the EASLM operates with monochromatic light. By having the image from the EASLM monochromatic the image transfer means can be relatively simple. Colour display may be achieved by producing sequential images from the OASLM in synchronisation with passing the image formed on the OASLM through a particular colour filter.